Electric machines have a stator and a rotor; the stator has axial slots that house transposed bars connected to each other to define a winding (typically a three-phase winding).
With reference to FIG. 12, each slot 8 houses two transposed bars 30 one on top of the other. Each transposed bar 30 has four stacks of strands 15; the four stacks of strands 15 define two side-by-side elementary transposed bars 31. The elementary transposed bars 31 have terminal lugs 32 (usually solid copper blocks) that are connected together to define the winding. As can be seen on the left of FIG. 12, reference B, the bars 30 with the lugs 32 end at different positions, see especially the two bars 30 on top at the left side. The same applies to the bars 30 with lugs 32 at the bottom of the left side, where the left bar 30 is longer. The left bar 30 at the top at the left side in FIG. 12 is shorter than the adjacent right bar 30, which is also referred to as crossover connection. This misalignment of the end pieces of the bars 30 originates from the transpositions of the strands 15 of the bars 30 in the state of the art. This configuration is critical in terms of stresses in the material and the welding process to weld the bars 30. At the right of FIG. 12, reference A, the bars 30 are shown when adjusted in length and the corresponding lugs 32 at the end of the bars 30 are connected together, the strands 15 of the bars 30 are not transposed.
The document U.S. Pat. No. 7,898,127 discloses an embodiment in which radially aligned lugs are reciprocally connected (this connection is similar to the connection identified by reference A in FIG. 12).
This embodiment has the problem that during operation it causes large losses, because the position of the elementary transposed bars is not exchanged.
In another embodiment of U.S. Pat. No. 7,898,127, in addition to radially aligned lugs similar to those identified by reference A in FIG. 12, also crossover connections between lugs 32 are provided (these connections are similar to the connections identified by reference B in FIG. 12). The crossover connections connect the lugs 32 of an elementary transposed bar at the right of a transposed bar with an elementary transposed bar at the left of another transposed bar; this way a position exchange of the elementary transposed bar is achieved.
This embodiment allows lower losses, but crossover connections are difficult and time consuming to assemble.